The demands of the seated work position mandate the user to accommodate a range of postural adjustments from the slightly rearward reclined rest position through to the forward hunched task posture. Passive automatic adaptation or adjustment of the seat support system is required if the natural balance and equilibrium of the body's support is to be maintained. Failure to maintain the body's equilibrium and structural balance will result in the creation of adverse, static postural loads and forces responsible for fatigue and biomechanical dysfunction so common in today's seated society.
There have been many attempts to better design a seating arrangement for persons working at a desk or computer terminal. Such ergonomic chairs are described, for instance, in U.S. Pat. No. 4,650,249 which issued on Mar. 17, 1987 to Cerber; U.S. Pat. No. 4,738,487 which issued on Apr. 19, 1988 to Shalinsky et al.; U.S. Pat. No. 5,048,893 which issued on Sep. 17, 1991 to Cowan et al. and Applicant's Canadian patent application Ser. No. 2,116,079 which was filed on Feb. 21, 1994 and laid-open on Aug. 23, 1994 in the inventors' names of Cowan et al. It has been found that when a person leans forward to work or back in a rest position, there is a movement combination of the person's body pivoting about the ankles of the person with the person's upper body pivoting about a center called the “H-point” which is a natural pivoting point of the torso and thigh lines. The H-point is defined in SAE standard J826. Although most chairs described in the above prior art provide reasonable adjustment in the fore and aft directions and allow for tilting of the seat, they do not provide, except in Canadian patent application Ser. No. 2,116,079, the combined movement of seats and backrests pivoting about the respective ankle point and the H-point and, therefore, result in a compromise in terms of vertical adjustment. An upward movement of any part of the chair will jeopardize the body's equilibrium and structural balance.
Cowan et al. describes, in Canadian patent application Ser. No. 2,116,079, a work station chair having a seat passively pivotable about the ankle of the person sitting on the chair and a backrest passively pivotable about the H-point. A cable system is provided for positive adjustment of resiliency of the pivoting movement of the seat and backrest for different weight loads of persons sitting on the chair. However, the H-point is physically required in the chair, which is a pair of pivoting pins attached to two arm support posts respectively. Such a configuration limits the application of the H-point backrest because the position of the H-point is always above the seat and cannot be attached to the supportive structure under the seat. Therefore, an improvement is desirable. A positive adjustment mechanism simpler in structure and easier for use is also desired to replace the cable system which has to be adjusted in an inconvenient rear position.